This invention relates to methods of site remediation for mitigating contamination from metals or metal compounds, particularly resulting from the situation of mine tailing waste on or near water sources such as streams or underground aquifers.
In the mining industry milling of mined rock is frequently done proximate to water supplies, since typically large quantities of water are used in the milling process to develop a suitable concentrate for refining operations. Mills are often centrally located to receive ores from a variety of different mine sources such that the composition of discarded material is often complex, having a variety of different potential contaminants. During the milling operation, ore is customarily crushed to a fine consistency, such that discarded tailings include primarily fine sands and slimes. Slimes are generally characterized as material of a particle size that remains suspended in water for a period of time and are usually less than 200 mesh in screen size.
Ore dressing circuits that were designed to obtain a concentrate have historically used water as a carrier medium for classification of material according to size or separation of material according to specific gravity. After separation to recover a majority of the valves from the remaining gangue, the gangue is discharged in tailing dumps either as a slurry or in a wet cake condition.
Since all are dressing circuits ore unable to recover one hundred percent of the target values from an ore, remnants of the target values remain in discarded tailings in concentrations of marginal economic value, particularly when using conventional recovery methods. In addition to target values, the tailing generally include minerals that were not originally targeted for recovery, but which pose hazards to water sources. Such minerals, frequently in the form of metal sulfides oxidize to water soluable sulfates on exposure to the elements including air, rain and underground seepage.
Current remediation techniques are usually directed to isolation of such tailing dumps, using impervious covers or caps to prevent influx of rain and to inhibit blow of fines. Containment may include underlying impervious base pads to isolate the dumps from ground water. Other similar techniques offer temporary solutions that do not resolve the potential problem, and are not truly remedial over the long run. Furthermore, such containment techniques take the site out of use and unnecessarily establish graveyards with all but minimal surface use excluded. Finally, such containment structures do not insure that avenues for escape of contaminants into the proximate water sources do not in fact occur. Eventually, contamination is inevitable and subsequent remediation costs are compounded by the containment structures put in place.